Synopsis
The episode's opening continues from the final scene of "Army of Ghosts"; Dr Singh (Raji James), Mickey Smith (Noel Clarke), and Rose Tyler (Billie Piper) are trapped in a sealed room within Torchwood's Canary Wharf. Four Daleks, accompanied by a device known as the "Genesis Ark", have emerged from the void ship. A Dalek Supreme called Dalek Sec extracts information about Earth from Singh, killing him in the process. He discovers that a separate invasion is in progress, and sends Dalek Thay to investigate. The Cybermen, who took control of Torchwood, detect the Dalek technology and offer an alliance. The Daleks decline, and the two species declare war.
While discussing humanity with the Doctor (David Tennant), the Cyber Leader is destroyed by a strike team led by Jake Simmonds (Andrew Hayden-Smith), from the same universe as the Cybermen. Jake takes the Doctor to his universe and a parallel Torchwood. Pete Tyler (Shaun Dingwall) tells him that the Cybermen vanished and they have followed them. The breach is causing unprecedented global warming on the parallel Earth and must be closed before the damage destroys both worlds. They return to Earth and approach the Cybermen with a truce against the Daleks.
Meanwhile, Rose surmises that they were kept alive because, as time travellers, their touch would activate the Ark. Sec explains that they cannot open the Ark because it is stolen Time Lord technology. He demands that Rose open it, but she refuses, and mocks the Daleks until the Doctor appears. Upon realising that the Daleks are the enigmatic Cult of Skaro, he uses his sonic screwdriver to allow the Cybermen to attack the Daleks, letting the humans escape. Mickey accidentally activates the Ark when escaping, and the Daleks, after escaping the Cybermen, travel to the exterior of Canary Wharf to release the Ark's contents: millions of Daleks who were imprisoned during the Time War. These Daleks encounter the Cybermen, and the two races begin fighting all over the world.
The Doctor and his companions flee into the tower. En route, Pete rescues Jackie from upgrading and the couple are reunited. As the Dalek–Cyberman war rages outside—with humans being massacred in the crossfire—the Doctor brings everyone to the breach room. He explains that crossing the Void causes a traveller to become saturated in Void material. If he opens the breach to the void, any being saturated will be pulled in. He will then close the breach forever. However, Rose and Mickey have crossed the Void and are contaminated and vulnerable.
The Tylers and Mickey cross into the parallel universe, but Rose decides she would rather be with the Doctor than her family and jumps back to help him. After being bought some time by an upgraded Yvonne, who has managed to regain her humanity, they open the breach and hold onto a pair of magnetic clamps as the Cybermen and Daleks are pulled in, though the Cult of Skaro escape via an "emergency temporal shift". Rose's lever slips; she pushes it back into position but loses her grip. Before she falls into the Void, Pete reappears, catches her, and jumps back to his world. The breach closes and leaves a devastated Rose trapped in the other world.
Some time later, Rose has a dream where she hears the Doctor's voice calling her. The Tyler family follow the voice to a remote bay in Norway called Bad Wolf Bay, where an image of the Doctor appears; he is harnessing the power of a supernova to transmit through one of the final breaches. Because the breach is to close permanently in two minutes, the pair share one last conversation. Rose breaks down in tears and tells him that she loves him, but as the Doctor starts to reply, the breach closes; once he has disappeared, Rose begins to cry and Jackie sprints to her daughter, meeting her in a hug in an attempt to console her upset child. In the TARDIS, a tearful Doctor regains his composure and sets a new course. He looks up to see a woman in a wedding dress (portrayed by Catherine Tate).
Read more about this topic: Doomsday (Doctor Who)